The present invention relates to a method and device for exciting a plasma using microwaves, at the electronic cyclotronic resonance, the object being to produce a homogeneous plasma of large volume and high density.
The production of plasma can be used in a varied number of applications such as surface treatments or the production of ion beams.
Most of the plasma-exciting techniques applied in laboratories and for industrial purposes use continuous discharges or radio-frequency discharges of which the coupling is generally capacitive.
Various excitation devices are known for the very specific field of microwaves.
A first type of excitation device uses a coupling inside a microwave cavity, this producing plasmas at relatively high pressure (10.sup.-1 to 10.sup.2 Pa) and confined in volumes of small dimensions.
In a second type of excitation device, the coupling is performed by surface wave (surfatron, surfaguide). In this case, the accessible range of pressure is wide (10.sup.-1 to some 10.sup.5 Pa.). In this case generally, columns of plasma of small diameter are obtained.
Excitation of the plasma may also be obtained by coupling in magnetic field at the electronic cyclotronic resonance frequency. In this case, the resonance is obtained for a magnetic field B and an excitation frequency f, linked by the relation: ##EQU1## wherein m and e are the mass and the charge of the electron. By way of example, for a frequency of 2.45 GHz, it is necessary to use a magnetic field of 875 Gauss in order to obtain the resonance.
With this particular type of excitation, plasmas can be obtained at very low pressure, around 10.sup.-2 Pa.
An excitation of plasma has also been achieved with a "Lisitano" type antenna. The plasma can be ignited if the antenna is immersed in a uniform magnetic field large enough to obtain the electronic cyclotronic resonance.
The different excitation devices indicated hereinabove all present certain problems of adaptation for certain applications, especially when it is necessary to have large volumes of homogeneous plasmas of high density in a wide range of pressure. Such requirements exist in particular, for the treatments of surfaces by plasma, in metallurgy as well as in microelectronics, for depositing, engraving or excitation operations.
It has been proposed, for example in French Patent Application No. 83-10116, to carry out a multipole magnetic confinement and to use a localized source of microwaves in order to obtain a plasma with a good homogeneity. Such a device however requires that the average free path of the ions be about equal to if not greater in size than the plasma. This imposes to have to work at low pressure, about 10.sup.-1 to 10.sup.-3 Pa. Therefore the electronic cyclotronic resonance is used in this case to excite such plasmas. But there is one important restriction which is the density of plasma which can be obtained with such a device. Indeed, an increase in the plasma density implies, either an increase in the surface of the microwave source, or an increase in the number of sources.
These two solutions however come up against the problem of producing a large volume of magnetic field by using coils. Besides the cost of the power supply, there is a restriction in that the presence of high stray fields, away from the coil, make the plasma anisotropic. It is, moreover, necessary to introduce the microwave electric power in vacuum through windows in dielectric materials, such as quartz or alumina, which raises technological problems in certain applications. Finally, the presence of a residual microwave power in the plasma can prove to be a problem with certain techniques such as engraving with resin masks.
It is not possible to increase the density of the source by increasing the microwave frequency. Indeed, in this case, the magnetic field necessary to the resonance is also greatly increased, which increases the problems mentioned hereinabove. In addition, at the high frequencies in dominant mode, the gain is reduced, as is the cross-sectional surface of the waveguide.